The present invention is related to a spot scanning system wherein laser generated scanning beams are reflected from a multi-faceted rotating polygon onto a photosensitive image recording medium. More particularly the invention is directed to methods for predicting the facet deformation produced at relatively high rotational speeds and using the predicted deformation characteristics to improve the efficiency of the scanning system.
Raster output scanners (ROS) are finding increasing use in high speeds xerographic machines such as copiers/duplicators and printers. Typical prior art system are shown, for example, in U.S. Pat. Nos. 4,040,096 and 4,884,083. These prior art ROS system utilize a plurality of optical elements to generate and transmit modulated beams of light along an optical axis forming a modulated beam which scans across a photosensitive recording medium as a series of scan lines forming a latent electrostatic image thereon. A key component in all of these systems is a rotating polygon having a plurality of reflective mirrored surfaces, or facets. The modulated light beam output of the coherent light source, which may be a gas or diode laser, is directed across the multi-faceted polygon and light is reflected from each facet and directed as a scanning beam across the surface of the recording medium. As these ROS systems have evolved, faster printing speeds have been enabled by increasing the scan speed, necessitating the polygon to be driven at higher rotational speeds. These higher speeds have inherent image quality problems which hitherto have not been fully appreciated. At these rotational loads, it has been found that the radial surface of the facets of the rotating polygons undergo a centrifugal-force-induced deformation which, while relatively of small magnitude, may still have undesirable effects on the quality of the image being formed at the photosensitive surface. In other words to the extent that this deformation departs from the desired flat, or planar, reflecting surfaces the image formed by the reflected scanning beams at the recording surface is adversely affected. While this deformation induced error is quite small, it could absorb a significant percentage of the facet flatness tolerance allowed for a given polygon scanner.
The present invention is, therefore, directed to an analysis of the type and extent of the facet deformation produced in various polygon designs and towards techniques for compensating for this deformation, the deformation being predictable for specific system designs. According to a first aspect of the invention, radial facet distortion curves are analytically determined and plotted establishing the type of deformation or departure from a perfectly planar orientation which results at specified rotational speeds and for varying numbers of polygon facets. It is then demonstrated that the polygon facet surface for the selected design can then be manufactured or processed so as to create the reverse deformation curves in the facet surfaces so that, at the rotational loads, the deformation will produce the desired flatness at each facet surface.
According to another aspect of the present invention, a prediction of deformation characteristics is utilized to compensate, not only for the errors which would be created by the departure from flatness of each facet, but also to design the polygon geometry so that the deformation will produce a radial curve on the facet surface which can perform the function of a lens element such as an f.theta. lens which has hitherto been required to be placed in the optical path between the polygon and the recording medium. More particularly, the invention is directed towards a raster output scanning system comprising; means for generating a beam of high intensity, modulated coherent light, optical means for imaging said beam to a spot at the surface of a photosensitive image recording medium, a multifaceted polygon located in the path of said imaged beam between said beam generating means and said medium and having reflective facet surfaces for reflecting incident beams onto the medium, means for rotating said polygon at a rotational speed so that the reflected light is scanned as successive scan lines across said medium, the improvement wherein the surface of said facets at rest, have a predetermined non-flat surface which is distorted at said rotational speed, so as to assume a flat orientation.